1. Field of the Invention
This invention pertains generally to electrical switching apparatus and, more particularly, to circuit breakers that detect electrical conductivity faults, such as loose electrical connections, of a power circuit. The invention also pertains to a method for detecting electrical conductivity faults.
2. Background Information
Electrical switching apparatus include, for example, circuit switching devices and circuit interrupters such as circuit breakers, receptacles, contactors, motor starters, motor controllers and other load controllers.
Circuit breakers are generally old and well known in the art. An example of a circuit breaker is disclosed in U.S. Pat. No. 5,341,191. Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. Molded case circuit breakers, for example, include at least one pair of separable contacts which are operated either manually by way of a handle disposed on the outside of the case or automatically by way of an internal trip unit in response to an overcurrent condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
Arcing is a luminous discharge of electricity across an insulating medium, usually accompanied by the partial volatilization of electrodes. An arc fault is an unintentional arcing condition in an electrical circuit. Arc faults can be caused, for instance, by worn insulation between adjacent bared conductors, by exposed ends between broken conductors, by faulty electrical connections, and in other situations where conducting elements are in close proximity. Arc faults in systems can be intermittent since the magnetic repulsion forces generated by the arc current force the conductors apart to extinguish the arc. Mechanical forces then bring the conductors together again in order that another arc is struck.
During sporadic arc fault conditions, the overload capability of the circuit breaker will not function since the root-mean-squared (RMS) value of the fault current is too small to activate the automatic trip circuit. The addition of electronic arc fault sensing to a circuit breaker adds one of the elements required for sputtering arc fault protection—ideally, the output of an electronic arc fault sensing circuit directly trips and, thus, opens the circuit breaker. See, for example, U.S. Pat. Nos. 6,710,688; 6,542,056; 6,522,509; 6,522,228; 5,691,869; and 5,224,006, which deal with DC and AC arc fault detection. See, also, U.S. Pat. No. 6,720,872, which deals with a receptacle.
A glowing contact is a high resistance connection, which can form at the interface of a copper wire and a screw terminal, for example, of a receptacle. The resulting temperature rise at this connection point can melt the wire's insulation and damage the receptacle. High resistance connections, such as cause glowing contacts, are most typically “behind the wall” and, thus, are hidden. Hence, it is desirable to be able to detect this condition and interrupt the current before the glowing contact fault progresses to a hazardous condition. See, for example, U.S. Pat. No. 6,707,652.
U.S. Pat. No. 5,608,328 discloses that widespread methods for precisely locating faults in power cables are based on acoustic detection of an arc at the fault. Typically, a surge generator or “thumper” is used to excite the power cable with a series of high-energy electrical pulses which, in turn, prompt audible sparking and vibration at the fault. U.S. Pat. No. 5,608,328 discloses that a series arc, once formed, tends to grow in length by reason of the thermal and electrochemical action of the arc. The arc literally erodes the adjacent contacts thereby assuring, absent human intervention, that the once marginal “opening” will become a full-fledged gap. This gap will continue to sustain an arc for hours or even months until it grows beyond an arc-sustaining maximum. During such periods, electrical and acoustic noise will be produced by the arc. Further, substantial energy will be generated by reason of the volt-amp product associated with the gap/arc which must be dissipated in order to maintain temperatures within safe limits. The arc is detected by detectors that receive electrical radio frequency (RF) noise.
U.S. Pat. No. 6,734,682 discloses a portable arc fault locating and testing device that employs an ultrasonic pick-up coil and an ultrasonic detector in combination with an audible pick-up coil and an audible detector. A circuit determines the correlation between the ultrasonic sound and the audible sound characteristics of an arc fault.
U.S. Pat. No. 6,777,953 discloses a system for locating parallel arcing faults in a set of wires. The system includes a handheld ultrasonic monitor to measure and indicate the distance from the operator to the arc. It measures both the electromagnetic pulse from the arc and the ultrasonic emission from the arc and uses the difference in arrival times to calculate the distance to the arc.
U.S. Pat. No. 6,798,211 discloses a fault distance indicator that locates a fault in a power line by modeling pulses of reflected traveling wave signals which are generated from electrical arcs that occur as a result of the fault. The fault distance indicator is mounted directly on a power line within a transformer enclosure, is powered by a power signal obtained from a transformer secondary and includes a transceiver, such as an infrared transceiver, although radio frequency or ultrasonic transceivers may be used.
U.S. Patent Application Publication No. 2003/0037615 discloses the generation and detection of acoustic guided waves to evaluate the condition of insulation on electrical wiring. For example, suitable transmitter and receiver transducers are broadband acoustic emission piezoelectric transducers. Signal transmission occurs at one location on the electrical wire to be evaluated and detection occurs at one or more locations along the electrical wire. The number and position of the detection locations depends on the user's preference. In one embodiment, transmission and detection occurs at one location, which is especially effective for evaluating the termination points of wire, such as at connectors, as well as for detecting signals reflected from flaws. For connectors, one transducer can be used to transmit the signal to the connector and detect the reflected signal. The transducer would be positioned as close as possible to the connector. In another embodiment, detection occurs at one or more locations separate from the transmitting location. Two simultaneous measurements can be taken to generate both attenuation and speed values.
The web site at http://www.idiny.com/chafing.html states that a wire chafing sensor is a passive solution to the problem of wire chafing detection by listening to noise signatures in the wire. This also states that the system can detect wire chafing, arcing and burning, and that pattern recognition software categorizes degrees of chafing.
One prior proposal for loose electrical connection detection utilizes relatively high voltage pulses to detect the associated break that could occur across such a loose electrical connection. The problems associated with this proposal include: (1) the possibility of damage to the corresponding electrical circuitry; and (2) the possible introduction of a relatively high noise-to-signal ratio in the electrical circuit, which may distort the true signal. Also, system stimulation and response monitoring may miss many loose intermittent events.
There is room for improvement in electrical switching apparatus, such as, for example, arc fault circuit breakers and receptacles, and in methods for detecting electrical conductivity faults.